1. Technical Field
The present disclosure generally relates to methods and processing in the field of seismic data acquisition, and particularly to the acquisition and processing of seismic data.
2. Background Information
Seismic surveys image or map the subsurface of the earth by imparting acoustic energy into the ground and recording the reflected energy or “echoes” that return from the rock layers below. The source of the acoustic energy is usually generated by an explosion or seismic vibrators, or air guns (and marine vibrators) in marine environments.
During a seismic survey, the energy source is positioned on or near the surface of the earth. Each time the energy source is activated it generates a seismic signal that travels into the earth, is partially reflected, and, upon its return, may be recorded at many locations on the surface as a function of travel time.
The sensors that are used to detect the returning seismic energy usually take the form of sensors like geophones or accelerometers (land surveys) and hydrophones (marine surveys). The returning seismic energy is acquired from a continuous signal representing displacement, velocity or acceleration that may be represented as an amplitude variation as a function of time.
Multiple source activation/recording combinations are subsequently combined to create a near continuous image of the subsurface. A survey produces a data volume that is an acoustic image of the subsurface that lies beneath the survey area.
A seismic vibrator generally takes the form of a truck or other vehicle that has a base plate that can be brought into contact with the earth. A reaction mass in association with a baseplate is driven by a system to produce vibratory motion which travels downward into the earth via the base plate. A similar system of actuating devices operates in the marine vibrator. A survey may be designed that uses multiple vibrators, each being activated simultaneously so that the recording instruments capture a composite signal with contributions from all of vibrators. The composite signal forms a separable source vibrator record that allows for source separation through data inversion.
One vibratory seismic data acquisition method for acquiring separable source vibrator records is known as high fidelity vibratory seismic. In this method, multiple seismic vibrators are operated simultaneously, thereby creating a complex source signal wherein separate source signals from separate vibrators or groups may be separated during subsequent processing.
In separable sweep applications the contributions of each individual vibrator from the recorded composite signal in a multi-vibrator survey may be separated. The sweep signals of each vibrator are varied in such a way as to make later separation feasible. This may involve the use of phase encoding of a constant phase shift to each vibrator's signal relative to another vibrator sweep in the group. When using multiple sweeps of the vibrators, a different phase encoding scheme may be employed for each sweep.
The fidelity of the source separation depends to a large degree on the selection of an appropriate vibrator sweep parameters, a good scheme being one that leads to better (meaning higher signal to noise) source separation. Better source separation, in turn, will result in an improved data quality.
The description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples or embodiments shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.